There has been extensively employed for various diagnoses an ultrasonic diagnostic equipment with which a subject is irradiated by an ultrasonic beam and an echo thereof is indicated by modulating the brightness of a cathode ray tube or the like so as to obtain an ultrasonic tomogram.
This invention relates to a method and apparatus for controlling ultrasonic waves which are radiated from or received by a phased array ultrasonic transducer for the electronic scanning of the ultrasonic beam employed in the ultrasonic diagnostic equipment or the like.
In order to deflect the ultrasonic beam, there has heretofore been used a method in which a plurality of transducer elements put into an array are respectively endowed with differences in the time of a driving pulse voltage.
FIG. 1(a) shows the state of ultrasonic waves in the case where n (1 - n) elements rectilinearly arrayed are driven at the same time. The main beam of the ultrasonic waves exists orthogonally to the direction of array.
In case of FIG. 1(b), the elements 1 - n are driven by bestowing delay times .tau..sub.1 - .tau..sub.n (.tau..sub.n = d.sub.n /V, V: the velocity of sound in a medium to-be-observed) on the respective elements so that the wave fronts of sound waves may become in-phase at distances d.sub.1 - d.sub.n. In this case, the main beam of the ultrasonic waves lies in a place deflected by an angle .theta., .theta. = sin.sup.-1 (V.multidot..tau..sub.n /l) (l: the total length of the array ultrasonic transducer).
Likewise, when d.sub.1 &lt; d.sub.n (.tau..sub.1 &lt; .tau..sub.n), the main beam of the ultrasonic waves is deflected by -.theta. as shown in FIG. 1(c).
As described above, the deflection of the main ultrasonic beam can be caused by driving the respective elements of the phased array ultrasonic transducer with appropriate delay times. Similarly, receiving waves can be endowed with directivity by bestowing appropriate delay times on received signals from the respective elements and adding them on the side of receiving the ultrasonic waves.
An example of a prior-art circuit for bestowing the appropriate delay times on the transmitting or received signals is shown in FIG. 2. Since the illustrated circuit is generally employed for the reception, the case of the reception will be explained here. Electric signals which are generated from n elements by incident ultrasonic waves .tau..sub.i are delayed by variable delay circuits 1.sub.n - 1.sub.n and then added by adders 2.sub.1 - 2.sub.n connected between the adjacent ones of the elements, and are thereby endowed with directivity.
Assuming now that the elements are arrayed at equal intervals, .tau..sub.1 = .tau..sub.2 = . . . = .tau..sub.n .ident. .tau.. As the delay times of the elements, a value which is obtained by equally dividing the maximum delay time necessary for a desired angle of deflection by a value being smaller by one than the total number of the elements is given between the adjacent elements. Therefore, a single control signal suffices for the delay times.
Letting .DELTA. denote the gap between the respectively adjacent elements and .theta. denote the angle of deflection, the delay time .tau. is given by Eq. (1): EQU .tau. = (.DELTA..multidot.sin .theta./V) (1)
where V represents the velocity of sound in the medium. By way of example, if the gap .DELTA. between the elements is 0.5 mm, the angle of deflection .theta. is 45.degree. and the velocity of sound in the medium V is 1,500 m/sec in Eq. (1), then the delay time .tau. becomes 236 ns. Assuming now that .theta. is scanned at every 0.5.degree., .tau. need be changed at every 2 - 3 ns. However, when the elements adopted have a natural frequency of, for example, about 2 MHz, one wavelength corresponds to 500 ns, and it is practically meaningless to perform the phasing at such accuracy better than about 1%.
If, in this manner, the delay times are controlled by merely prescribing the accuracy of the phasing and quantizing the delay times, the above-mentioned condition .tau..sub.1 = .tau..sub.2 = . . . = .tau..sub.n is no longer satisfied, and the single control signal for the delay times becomes insufficient. Therefore, the control becomes complicated.